The present invention relates generally to a special class of wide bandwidth--yet not wide frequency bandwidth--communications systems which are spread polarization--yet not spread frequency spectrum. The invention utilizes a bandwidth--the polarization modulation bandwidth--which is presently under utilized for communications purposes. The invention, in one method of application, also relates to the use of this wide polarization bandwidth in systems requiring only small-sized, in contrast to large-sized antennas. The present invention, also describes a method for obtaining wide (polarization) bandwidth communications using dual-feed orthogonally polarized antennas, both of narrow frequency bandwidth, to provide a broad-polarization bandwidth for transmission/reception after Launch from the antennas. With respect to antennas, which transmit/receive waves of one polarization (unipolarized), and receivers unmatched in polarization to tile signals from systems of the present invention, such signals are spread polarization, as opposed to spread (frequency) spectrum.
It is an object of the present invention to provide a communications system which is dependent on two orthogonally polarized antennas to provide one polarization modulated wave.
It is a further object of the present invention to provide polarization modulation communications rather than the phase modulation communications of prior art which require only one broad-band antenna for transmit or receive, which are of one or few static polarizations, and which create and utilize large frequency bandwidths. Such an antenna of prior art which is of one or few static polarizations, whether linearly, circularly, or elliptically polarized, will be referred to as unipolarized.
It is a further object of the present invention to provide a spread polarization communications system. There are two modes of functioning of the present invention: the analog polarization modulation mode and the hopping polarization modulation mode. Unlike spread (frequency) spectrum systems of prior art, the spread polarization system of the present invention in the hopping mode is spread in polarization and temporal bandwidth, in contrast to frequency bandwidth. As a communications system of the present invention is spread in polarization, and as the majority of communications antennas of prior art are unipolarized (single polarization samplers), then the polarization modulated signals of the present invention will be spread polarization to such antennas.
It is a further object of the present invention to provide a communications system in which phase changes and modulations in the emitted signal occur between signals from two orthogonally polarized emit/receive antennas. This object can be contrasted with phase changes and modulations occurring in signals of prior art which are phase changes between two waves of the same polarization so that the resultant modulated wave is of the same polarization. In the case of polarization modulation in communications systems of the present invention, the phase changes occurring with phase modulation are between two waves (or antennas) of orthogonal polarization, and the resultant launched wave is of a changing (nonstatic) polarization and different from that of the two constituent waves (except under certain trivial conditions--e.g., one or other of the constituent waves being zero). A specific advantage in using phase modulation of orthogonally polarized waves producing polarization modulation and omnipolarized waves as in the present invention, as opposed to phase modulation of unipolarized waves producing unipolarized waves of prior art, is that as the conventional antenna systems of communications systems of prior art are unipolarized, polarization modulated communications systems of the present invention present a signal spread in polarization to reception by those conventional antenna systems. In contrast, phase modulated systems of prior art are unspread in polarization to other systems of prior art.
It is a further object of the present invention to provide a communications system in which signal rotation of the polarization axis with respect to a ground-plane is controlled, so that, together with polarization changes, two-dimensional trajectories on a Poincare sphere representation (FIG. 1.) are possible for the carrier signal.
It is a further object of the present invention to provide both analog and hopping polarization modulated communications systems. Analog polarization modulated signals of the present invention require at least one antenna which is broad band. Polarization hopping modulated signals are constituted of two individual signals which can be narrow in frequency bandwidth (although broad in polarization bandwidth). In the case of polarization hopping communications systems of the present invention: (i) there is minimum interference to other receivers; and (ii) narrow-band and small antennas can be used. There are many advantages to using small, miniaturized antennas. Miniaturized antennas can be obtained by the application of dielectric-ferrite cladding (cf. Chatterjee, 1985; Fujimoto et al, 1987). However, short antennas obtained by these methods have disadvantages, one of which is a resulting high Q (narrow bandwidth). The present invention, in its polarization hopping mode, utilizes two short orthogonally polarized antennas of high Q but the polarization hopping creates a large polarization bandwidth sequentially.
The above and other objects of the invention are achieved in a communications system the signals of which can be represented as a path or changing locations or trajectories on a Poincare sphere (FIG. 1), where the lines of longitude refer to changes in polarization and lines of latitude refer to rotation. In this representation, a movement on the surface of the sphere can represent changes in both polarization and rotation (FIG. 2).
Means are described for generating spread polarization signals which are M-ary with respect to the carrier, and binary with respect to the signal. For example, in the polarization hopping mode of function, suppose at some instant that the carrier's instantaneous polarization is, e.g., 256.degree., and the carrier's instantaneous rotation is, e.g., 148.degree., then a "1" and "0" for the informational encoding is 256.degree.+90.degree. and 256.degree.-90.degree. at instantaneous rotation 148.degree.. At a subsequent instant the carrier's instantaneous polarization might be, e.g., 268.degree., and the carrier's instantaneous rotation might be, e.g., 156.degree., then a "1" and "0" for the informational encoding, now, is 268.degree.+90.degree. and 268.degree.-90.degree., respectively, at instantaneous rotation 156.degree.. Thus, in this instance, the polarization-rotation modulation method is M-ary with respect to the carrier, but binary with respect to the time domain informational sequencing.
Means are described for generating spread polarization signals which are M-ary with respect to the carrier, and M-ary with respect to the signal. For example, in the polarization hopping mode of function, suppose at some instant that the carrier's instantaneous polarization is, e.g., 256.degree., and the carrier's instantaneous rotation is, e.g., 148.degree., then the informational encoding is 256.degree.+i.degree. and 256.degree.-i.degree. at instantaneous rotation 148.degree.+j.degree. and 148.degree.-j.degree., i=x.sub.1, x.sub.2, x.sub.3, . . . degrees, j=y.sub.1, y.sub.2, y.sub.3, . . . degrees. At a subsequent instant the carrier's instantaneous polarization might be, e.g., 268.degree., and the carrier's instantaneous rotation might be, e.g., 156.degree., then the informational encoding, now, is 268.degree.+i.degree. and 268.degree.-i.degree., respectively, at instantaneous rotation 156.degree.+j.degree. and 156.degree.-j.degree., i=x.sub.1, x.sub.2, x.sub.3, . . . degrees, j=y.sub.1, y.sub.2, y.sub.3, . . . degrees. Thus, in this instance, the polarization-rotation modulation method is M-ary with respect to the carrier and also M-ary with respect to the time domain informational sequencing.
Furthermore, the signal of the present invention is spread not only in the time domain, i.e., CAR.sub.TD, but also in the polarization-rotation domain, CAR.sub.PR. This means that the same codes, e.g., direct sequence, linear congruence, quadratic congruence, optical, etc. (cf. Titlebaum, 1981; Titlebaum & Sibul, 1981; Bellegarda & Titlebaum, 1988, 1991a,b; Titlebaumet al, 1991; Drumheller & Titlebaum, 1991; Maric & Titlebaum, 1992; Maric & Titlebaum, 1992; Chung et al, 1989; Chung & Kumar, 1990; A et al, 1992), can be used in the polarization-rotation modulation technique as with conventional frequency hopping and time hopping systems. Therefore the polarization-rotation modulation technique of the present invention is more spread, or covert, than prior art.
A high processing gain PG is defined, as: EQU PG=BW.sub.RF /R.sub.info
where the RF bandwidth BW.sub.RF is the bandwidth of the transmitted spread signal; and R.sub.info is the data rate. Frequency and time spread spectrum signals produced by techniques of prior art achieve a large increase in BW.sub.RF by four methods: (1) using a frequency domain code, which increases the frequency bandwidth, BW(CAR.sub.FD), by hopping between narrow frequency bandwidth allotments, or by changing phase; or (2) using a time domain code, which increases the sequential bandwidth, BW(CAR.sub.TD), by hopping between narrow time bandwidth allotments, e.g., by pulse position modulation; (3) using an ultrashort pulse which is of wide frequency bandwidth, and (4) combinations of (1), (2) and (3) (cf. Simon et al, 1985; Dixon, 1984). The polarization-rotation technique of the present invention using trajectories in polarization-rotation space represented on the Poincare sphere permit an increase in another bandwidth, BW(CAR.sub.PR), so that the total bandwidth increase is given by: EQU BW.sub.RF =BW(CAR.sub.PR).times.BW(CAR.sub.TD).
For example, using conventional assumptions, in the case of prior art the RF bandwidth can be approximately 0.88 times the bandwidth-spreading code clock rate of, e.g., 10.sup.7 Hz. With a 3 KHz information rate, the process gain is BW(CAR.sub.TD)=(0.88.times.10.sup.7)/(3.times.10.sup.3)=2933.3 or 35 db. In comparison, if a polarization hopping system takes ten steps to traverse a trajectory on the Poincare sphere, if this is accomplished in ten changes in polarization and ten changes in rotation, and if the trajectory is repeated at a rate of 10 Hz, then the spread polarization-rotation bandwidth, BW(CAR.sub.PR), is 1000. If all other variables, e.g., the coding and code clock rates, are the same as in the case of systems of prior art, the total RF bandwidth is then 1000.times.BW(CAR.sub.TD), i.e., the polarization modulation PG is 65 db compared with 35 db for the case of systems of prior art. These modest figures are intended only for comparison--larger BW(CAR.sub.PR)s are achieved with faster polarization and rotation modulators--and similar exercises can be performed on other conventional codes. In all instances the polarization-rotation technique of the present invention will always provide superior processing gain.
Means are described for generating increased bandwidths and covert communications using advanced forms of the polarization-rotation technique of the present invention. Carrier trajectories (on the Poincare sphere) are not only obtainable with polarization-rotation modulations: d.phi./dt=constant; dr/dt=constant; but also with d.phi./dt=f(t); dr/dt=f(t); and also d.phi..sup.n /dt.sup.n =constant; dr.sup.n /dt.sup.n =constant; and d.phi..sup.n /dt.sup.n =f(t) and dr.sup.n /dt.sup.n =f(t), n=1,2, . . . This permits large numbers of codes and polarization-rotation spreading for the (polarization-rotation) carrier, CAR.sub.PR --quite apart from the codes for the time domain sequencing carrier, CAR.sub.TD.
In summary, the invention provides:
Communications with high processing gain and noninterfering communications utilizing unused bandwidths. PA1 Extremely spread polarization communications which are noninterfering with present spectrum users. PA1 Use of small, light-weight, narrow-band, antennas in the polarization hopping mode. PA1 Large numbers of noninterfering codes. PA1 Covert, low-probability of intercept communications. PA1 Low probability of decode if intercepted communications. PA1 1. Dual-feed orthogonally polarized antennas consisting of a referent antenna of narrow-frequency bandwidth together with a narrow bandwidth orthogonally polarized antenna providing a broad polarization-rotation modulation bandwidth. PA1 2. A relatively lossless, dispersionless delay line providing phase delays of fractions of a degree and cross-correlation capabilities to detect those fractions. Instantaneous phase changes over small intervals of time are transmitted and received by means of two small, high Q, antennas in the polarization hopping mode. In the analog continuous mode the instantaneous changes d.phi./dt or d.sup.n .phi./dt.sup.n are transmitted and received by means of one small high Q antenna and one larger, higher Q antenna. In the hopping mode the sampling and transmission is of the absolute discrete phase differences .DELTA..phi..sub.1, .DELTA..phi..sub.2, .DELTA..phi..sub.3, . . . at set intervals. PA1 3. A method of changing the orientation of one antenna with respect to the other, providing rotations, .psi., of the polarization plane. PA1 4. Four bandwidths are used in a system of the present invention: (i) the frequency bandwidth which is narrow; the (ii) polarization bandwidth which is wide and either continuous analog or polarization hopping; (iii) the rotation bandwidth; and (iv) the temporal or time hopping bandwidth. The high polarization sampling bandwidth permits a large number of discriminations, .DELTA..phi..sub.i, i.e., a large number of code elements. PA1 5. The channel signalling rate is determined by the Q of the antennas used. Correlator performance sets the length of the signal repertoire (.DELTA..phi..sub.1, .DELTA..phi..sub.2, .DELTA..phi..sub.3, . . . ). 20 Gigabits/sec. correlators are available with present technology. Thus the channel capacity of a communications system of the present invention is high because of both the signaling rate and also due to the high number of code elements. PA1 6. A communications system of the present invention can be used for indirect communications as well as point-to-point, provided that the polarization-rotation conditioning changes resulting from surface reflections are known. In the case of indirect communications, a test signal must be transmitted to a receiver to probe surface polarization conditioning-rotation so that receiver compensations in the carrier can be made.